I've used a custom marker icon and it works as expected, except on page load where there is a brief glimpse of the default Google red marker until the map is fully rendered, then it disappears and only the custom marker is visible.
I've created my maps using Google Maps API and they have custom JSON to change the colours of various elements. The maps are quite simple with a lot of extraneous information hidden, as we just want to show the locations of a business. I have created all the maps with views, there is a single map version showing just one location, and a common map that shows all the locations at once. Both types of maps have he same issue where the default map marker is briefly visible as the page loads.
I'm using Drupal 10, and Geolocation module version: 8.x-3.12
Active
3.12
Geolocation Field / Backend
Could you (separately) test v4?
Not quickly or easily, but it is possible if you think it's absolutely necessary.
I understand this is inconvienient, but due to limited resources, I won't work on this in v3 anymore.